The present invention relates to a process and to a unit for the MIG welding of aluminum and its alloys with double gas shielding.
In electric arc welding, MIG (Metal Inert Gas) welding processes are widely used for welding various metallic materials, such as carbon steels, stainless steels, aluminum, nickel, copper, titanium, zirconium and alloys thereof.
During the actual MIG welding, an electric arc is generated between an electrode, namely a meltable wire, and the workpieces to be welded together, so as to melt said meltable wire by the heat generated by the electric arc and thereby form a welded joint between the workpieces to be welded.
The metal constituting the welded joint therefore comes essentially from the meltable wire and, in part, from the workpiece edges undergoing melting along their line of joining, that is to say the xe2x80x9cmolten metalxe2x80x9d junction zone and the heat-affected zone lying on either side of this junction zone formed from xe2x80x9cmolten metalxe2x80x9d.
In order to avoid or minimize contamination of the welded joint being formed by atmospheric impurities liable to be present in the ambient air, it is common practice, in MIG welding, to shield all or part of the welded joint with a gas shield consisting of a gas or a gas mixture delivered by the welding torch used for implementing the process, that is to say for conveying and guiding the meltable welding wire coming from a reserve of wire or the like.
Thus, document U.S. Pat. No. 4,572,942 describes an arc welding process with meltable wire and gas shield, that is to say an MIG process, in which the gas shield is a quaternary mixture consisting of 40-70% argon, 25 to 60% helium, 3 to 10% carbon dioxide and 0.1 to 2% oxygen.
Furthermore, welding torches are also described in documents DE-A-2,143,466, U.S. Pat. No. 3,676,640, U.S. Pat. No. 3,155,811, U.S. Pat. No. 3,007,032 and U.S. Pat. No. 3,239,647.
Moreover, it has been demonstrated that improvements in the quality of the welded joint can be obtained by providing a double injection of shielding gas.
Thus, the document xe2x80x9cEffect of a two-layer annular flow of shielding gases on the process of consumable electrode weldingxe2x80x9d; Welding International 1996, 10 (10), 813-815 describes a double-gas-shielded welding torch, that is to say a torch comprising a central nozzle delivering a stream of argon and a peripheral nozzle, based coaxially with respect to the central nozzle, supplying a stream of carbon dioxide. In this case, the meltable wire is guided by a sheath located inside the central nozzle, that is to say the stream of argon flows on the outside of the wire feed sheath.
Similarly, the document xe2x80x9cMetal Transfer and Spatter Loss in Double Gas Shielded Metal Arc Weldingxe2x80x9d; Transactions of the Japan Welding Society, Vol. 19, No. 2, October 1988, pp 347-353 also describes a double-flow torch of this type used with the same gases.
However, although double gas injection does allow the quality of the weld to be improved it turns out that this technique considerably complicates the construction of the welding torch, given that it is necessary in this case for the torch to be provided with two concentric nozzles arranged coaxially around the feed sheath for the meltable welding wire constituting a consumable electrode.
In order to try to solve this problem of increased torch complexity, documents U.S. Pat. No. 4,731,518 and EP-A-480,837 teach the elimination of one of the two gas delivery nozzles and the introduction of the gas intended to form the central gas stream directly in the electrically conducting contact tube located at the end of the welding wire feed sheath, by introducing said gas either near and upstream of the torch, that is to say in the sheath, or much further upstream of the torch, that is to say at the upstream end of the wire feed sheath, or near it, or in the reserve of meltable wire, thereby allowing the wire in the entire wire feed sheath to be inserted.
In this case, the peripheral gas stream is normally delivered via a single delivery nozzle fitted around the wire feed sheath.
From these documents, any type of gas may be used, for example argon, carbon dioxide or helium, or mixtures of these gases with oxygen.
In the light of this prior art, the problem that arises is how to improve the known MIG welding processes so that aluminum or aluminum-alloy workpieces can be effectively welded by improving the penetration of the weld and the wetting of the weld bead, that is to say the angle of connection of the weld bead to the welded workpieces.
In other words, the present invention aims to further improve the quality of the welds obtained by an MIG process on aluminum and aluminum alloys.
The present invention therefore relates to an. MIG welding process for aluminum and aluminum alloys employing a welding torch comprising a torch body provided with a gas delivery nozzle and with a contact tube fitted into the torch body and emerging inside said delivery nozzle, said contact tube being electrically conducting and a metal consumable electrode passing through it, in which process:
a) a first gas stream is made to flow through said contact tube and a second gas stream is made to flow through said delivery nozzle, said first and second gas streams being directed toward the welding zone and constituting a gaseous shielding atmosphere shielding at least part of said welding zone, said second shielding gas stream being distributed approximately peripherally with respect to said first shielding gas stream so as to obtain two approximately concentric gas streams,
b) at least one electric arc is created between said consumable electrode and said welding zone in order to melt part of said metal consumable electrode and to deposit, in the welding zone, at least some of said molten metal intended to form at least one welded joint,
wherein
the first gas stream consists of argon, oxygen or a mixture of argon and oxygen, and
the second gas stream consists of argon or a mixture of argon and helium.
Depending on the case, the process of the invention may comprise one or more of the following characteristics:
the first gas stream consists of argon and the second gas stream consists of argon;
the first gas stream consists of oxygen and the second gas stream consists of argon or a mixture of argon and helium;
the first gas stream consists of a mixture of oxygen and argon and the second gas stream consists of argon or a mixture of argon and helium,
the first gas stream consists of a mixture of oxygen and argon containing at most 5% oxygen;
the second gas stream consists of a binary mixture of argon and helium containing from 20 to 80% argon and the rest being helium;
the first gas stream consists of a binary mixture of argon and oxygen containing from 0.01 to 2% oxygen and the rest being argon, preferably the mixture contains less than 2% oxygen and the rest being argon;
the ratio of the flow rate of the first gas stream to the flow rate of the second gas stream is from 0.1/15 to 4/15, preferably from 0.03/15 to 0.5/15;
the consumable electrode is a meltable wire, preferably a solid wire;
the first gas stream is introduced into the consumable-electrode feed sheath at a site located immediately upstream of the torch body or at a site located near the upstream end of said electrode feed sheath.
Moreover, the invention also relates to an MIG welding unit capable of implementing a process according to the invention, comprising:
a welding torch comprising a torch body provided with a gas delivery nozzle and with a contact tube emerging in said delivery nozzle through which the contact tube passes a wire-shaped metal consumable electrode;
electric current supply means for supplying at least said torch with electric current;
at least one first source of a first gas consisting of argon, oxygen or a mixture of argon and oxygen;
at least one second source of a second gas consisting of argon or a mixture of argon and helium;
at least one supply of meltable wire constituting the consumable electrode;
wire feed means allowing the meltable wire to be guided between said supply of meltable wire and said torch body, said wire feed means comprising at least one sheath forming a sleeve around at least part of said wire and within which sheath said meltable wire is guided;
first gas conveying means for conveying the first gas stream between said first source of said first gas and a site located upstream of the torch body or near the upstream end of said sheath surrounding the wire;
second gas conveying means for conveying the first gas stream between said second source of said second gas and the gas delivery nozzle with which the torch body is equipped.
The invention will now be explained more clearly with the aid of examples and with reference to the appended figures, given by way of illustration but implying no limitation.